Renal failure is increasing at the rate of 6% with a cost of 16 billion dollars per year;thus developing new therapies for kidney disease is of paramount importance. In the previous renewal, we demonstrated that a loss of constitutive expression of vascular endothelial growth factor (VEGF) occurs in several models of renal disease, and that replacement with VEGF could slow progression via its ability to stimulate renal capillary repair. In this proposal we continue our studies of the physiology and pathophysiology of VEGF in renal disease. In Aim 1 we will test the hypothesis that the constitutive expression of VEGF in specific tubular cells (collecting ducts and medullary thick ascending limb cells) plays a critical trophic role for the renal (peritubular) capillaries in development and in the adult animal. This will be tested by selectively knocking out VEGF in these tubular cells using the Cre-loxP approach. In Aim 2 we will test the hypothesis that VEGF administration may not be beneficial when endothelial NO levels are low, and in fact may accelerate vascular disease via its effects on monocytes and vascular smooth muscle cells. This will be tested by examining the effect of long-term VEGF expression (by gene transfer using the AAV vector system) in renal diseases in which endogenous NO levels are reduced or maintained. In Aim 3 we will test the hypothesis that the specific VEGF receptors may govern whether VEGF stimulation is good or bad, in that stimulation of VEGFR-1 is expected to exacerbate renal vascular injury whereas stimulation of VEGFR-2 should accelerate capillary repair and renal recovery, independent of the status of the NO system. This will be tested by the overexpression of VEGF mutants specific for each receptor in renal diseases in the presence or absence of NO blockade. Documentation of the role of VEGF in the normal kidney, the importance of the NO system in mediating the responses to VEGF, and the specific role of each receptor in this process should provide the key information to help guide future studies for the use of VEGF as a novel treatment of kidney disease.